Rosin ester-based resins are widely used for alcohol and water-based ink and coating formulations. In order for the resins to be suitably soluble in polar protic solvents such as water and/or alcohol, the resin must have a high degree of residual reactive functionality. The degree of residual reactive functionality of the resin is related to the number of unreacted carboxylic acid and hydroxyl sites on the molecule after the reaction of an adduct of rosin with an alcohol. In general, the greater the degree of reactive functionality, the more soluble the resin is in polar protic solvents.
Typically, rosin ester-based resins of this type are made in small scale batch-type reactions. Small scale reactions are used in order to maintain sufficient control of the reaction progress so that resins with the desired degree of functionality are produced. In general, the reaction is conducted at elevated temperatures and at atmospheric pressure by first reacting rosin with an unsaturated carboxylic acid to form a rosin adduct, then reacting the rosin adduct with a polyol to form a partially esterified resin. During the latter reaction, water is formed and is constantly removed from the reaction medium as it is formed, normally with the aid of an inert gas purge.
In an attempt to maintain the desired degree of esterification, the reaction mass is quickly cooled at a certain point in an effort to limit any further esterification of the rosin adduct. However, as long as the temperature of the reaction mass remains sufficiently high, further esterification of the adduct will continue with a concomitant loss in residual functionality. While smaller scale reactions allow for better control of the esterification reaction, it is quite difficult to terminate the reaction at precisely the desired point. Furthermore, efforts to stop the reaction typically involve solidifying or flaking the resin and therefore require the end-user to solubilize the flaked resin which is energy intensive and time consuming.
The high temperatures involved in production of partially esterified rosin may also create other problems. In addition to losses in functionality, exposure to elevated temperatures subsequent to the formation reaction will typically increase the molecular weight of the resin thereby increasing the potential for gelation of the resin and loss of flowability. Higher molecular weight increases solution viscosity and may further limit solubility. Gelation renders the resin useless.
For the foregoing and for other reasons, conventional resin production techniques are significantly deficient for producing a rosin ester resin having the desired degree of residual functionality. As noted above, the highly reactive nature of the product subsequent to the esterification step has encouraged the use of relatively small scale reactions which cannot take advantage of the economies of scale realized with larger scale reactions. Furthermore, additional processing steps are required to prepare resin solutions of alcohol and/or water-amine in order to prepare ink formulations containing the resin.
Accordingly, it is an object of the invention to provide a method for making highly functionalized rosin ester resins for ink compositions.
Another object of the invention is to provide a method of the character described which enables improved control over the properties of the final resin product.
A further object of the invention is to provide a method for producing a partially esterified rosin ester resin having a target acid number.
Yet another object of the invention is to provide a method for making partially esterified rosin esters which enables the use of large scale batch reactions.
Still another object of the invention is to provide a method for making a partially esterified rosin-based resin solution directly from the rosin ester reaction mass without an intermediate processing step.
A still further object of the invention is to provide a method of the character described which is economical and relatively simple to carry out.